PROJECT SUMMARY/ABSTRACT The overall goal of this proposal is to evaluate the efficacy of testosterone replacement therapy in improving fatigue and other outcomes such as sexual function, quality of life, body composition, muscle strength and physical activity in a double-blind, randomized, placebo-controlled trial in young cancer survivors who report fatigue and have testosterone deficiency. Fatigue is one of the most prevalent and debilitating symptoms in men with cancer affecting 70-100% of patients irrespective of their age. Cancer-related fatigue is experienced by patients not only during active cancer treatment, but is also highly prevalent in cancer survivors who exhibit persistent fatigue months to years after the end of their treatment with the highest prevalence being in recipients of chemotherapy and/or radiation therapy. In addition to fatigue, sexual dysfunction is also highly prevalent in male cancer survivors. Male cancer survivors also have increased fat mass and decreased lean body mass, a phenotype that predisposes them to reduced muscle strength. This phenotype of fatigue, sexual dysfunction and adverse body composition is commonly encountered in non-cancer patient populations with testosterone deficiency, a condition which is also highly prevalent (50-90%) in cancer survivors. Pivotal trials of testosterone replacement therapy in non-cancer patient populations have shown an improvement in fatigue, sexual function and body composition in men randomized to testosterone compared with placebo. However, the efficacy of testosterone replacement therapy on cancer-related fatigue has not been studied. We propose a large, 9-month, double-blind, randomized, placebo-controlled trial in which: 1) we will compare the efficacy of weekly testosterone injections versus placebo in improving fatigue scores in cancer survivors who report fatigue and also have testosterone deficiency, 2) we will compare the effects of testosterone replacement versus placebo on sexual function, well-being, mood and quality of life, and 3) we will determine the effect of testosterone administration on body composition, muscle strength and physical activity compared with placebo. With an increase in longevity among cancer survivors and the growing recognition of the importance of symptom control in the lives of these patients, the proposed trial is likely to have a significant clinical impact in the management of patient-reported outcomes. If testosterone therapy is found to be efficacious in alleviating these symptoms, the clinicians will have available to them a therapeutic option that is relatively inexpensive, convenient to use and has a good safety record in young men.